Impact of Smoothness on Fuel Efficiency

National Center for Asphalt Technology

Overview

• Background

• Objectives

• Factors Affecting Rolling Resistance (RR)

• Pavement Properties Affecting RR

• Conclusions

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Fuel Economy

• Vehicle aerodynamics (wind resistance)

• Engine efficiency (internal resistance)

• Tire-road contact (rolling resistance)

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Effect of Vehicle Speed

• 30 mph

– Rolling Resistance: 50%

– Internal Friction: 25%

– Air Drag: 25%

• 60 mph

– Rolling Resistance: 30%

– Internal Friction: 20%

– Air Drag: 50%

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Objective and Scope

• What pavement properties have greatest influence on rolling resistance & fuel economy?

• Synthesis of prior research and simulations

• 20+ studies included in literature review

• Experience on NCAT Pavement Test Track

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Rolling Resistance

• Force required to keep an object (i.e. wheel or tire) moving

– Energy losses on pavement surface

– Internal friction loses

– Energy losses from tire deformation

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Factors Affecting Rolling Resistance

• Material composition of tire

• Tire’s geometry

• Road composition

• Roughness of tire and road

• Ambient air temperature

• Vehicle speed

• Tire pressure

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Factors Affecting Rolling Resistance

• Material composition of tire

• Tire’s geometry

• Road composition

• Roughness of tire and road

• Ambient air temperature

• Vehicle speed

• Tire pressure

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Pavement Properties Affecting RR

• Pavement Type

• Deformation

• Smoothness

• Texture

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Pavement Type

Pavement Type

Rolling Resistance Coefficient

Concrete 0.010 – 0.020

Asphalt 0.012 – 0.022

Dirt 0.025 – 0.037

Sand 0.060 – 0.150

• Asphalt and concrete had similar rolling resistance coefficients

• Dirt and sand roadways are less fuel efficient than flexible and rigid pavements

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Walter and Conant (1974)

Pavement Deformation

• Deformation = Energy Loss

– Tire and Pavement

• Modeling

– Two springs in a series

– Effective stiffness dominated by softer material

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Pavement Deformation

• Pavement versus tire stiffness

• Energy lost from pavement deflection

Pavement Smoothness

• International Roughness Index (IRI)

– Driver perceived roughness

• How does smoothness affect rolling resistance/fuel economy?

• Affects wear and energy loss in shock absorbers

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Effect on Rolling Resistance

• Road 1

• Vehicle Speed: 50 mph

• IRI: 60 in/mile

• RR Coefficient: 0.012

• Road 2

• Vehicle Speed: 50 mph

• IRI: 120 in/mile

• RR Coefficient: 0.01272

• 6% increase in rolling resistance

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Effect on Fuel Economy

• Rolling Resistance versus Fuel Economy

– 10% increase in RR = 3-4% reduction in fuel economy

• At 60 mph, passenger vehicles

– 1.8 to 2.4% increase in fuel economy

• WesTrack

– 10% reduction in IRI

– 4.5% reduction in fuel consumption

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Pavement Texture

• Affects energy loss due to wear and small scale tire contact

• Megatexture

– 50 mm – 500 mm

• Macrotexture

– 0.5mm – 50 mm

• Microtexture

– < 0.5 mm

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Effect on Rolling Resistance

• Road 1

• Vehicle Speed: 30 mph

• MPD: 0.5 mm

• RR Coefficient: 0.012

• Road 2

• Vehicle Speed: 30 mph

• MPD: 1.5 mm

• RR Coefficient: 0.01404

• 17% increase in rolling resistance

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5.1% increase in fuel economy

Effect on Fuel Economy

• Sandberg (1990)

• Testing parameters

– Volvo 242

– Twenty road surfaces

– Three speeds

• Macrotexture = 5% change in fuel economy

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Effect on Rolling Resistance

• Road 1

• Vehicle Speed: 60 mph

• MPD: 0.5 mm

• RR Coefficient: 0.012

• Road 2

• Vehicle Speed: 60 mph

• MPD: 1.5 mm

• RR Coefficient: 0.0156

• 30% increase in rolling resistance

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9% increase in fuel economy

Most Influential Parameters

• At low speeds

– Smoothness and megatexture

• At high speeds

– Smoothness, megatexture, and macrotexture

• Smoothness and texture have most overall influence

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Conclusions

• Stiffness plays minimal importance in rolling resistance and fuel economy

• Smooth pavements can increase fuel economy between 1.8 and 4.5%

– More???

• Limiting texture could improve fuel economy

– Magnitude depends on speed

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NCAT Pavement Test Track

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Questions ?

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